1. Field of the Invention
The present invention relates to a solid-state imaging device which is suitable for use, for example, as a solid-state imaging device for detecting ultraviolet light and to a method of fabricating the same.
2. Description of the Related Art
In a conventional CCD solid-state imaging device, an on-chip lens and a planarizing film underlying the on-chip lens are generally composed of organic materials. For example, a photoresist film is used for the planarizing film and the on-chip lens, and an organic material, such as casein, is used for color filters between the planarizing film and the on-chip lens.
Since the organic materials described above do not transmit ultraviolet light, the device is not sensitive to ultraviolet light.
Therefore, it is not possible to form an on-chip lens if a CCD solid-state imaging device having the conventional structure is required to be sensitive to ultraviolet light.
If the on-chip lens is not formed, since light entering an area in which a shading film is formed in the vicinity of a photosensor cannot be detected, the effective aperture ratio of the sensor section is decreased, resulting in a reduction in sensitivity.
In view of the problem described above, for example, in order to improve the sensitivity to ultraviolet light, solid-state imaging devices having structures described below have been proposed.
1. A structure in which an organic film above the sensor section is eliminated or the thickness of the organic film is significantly decreased in a conventional CCD solid-state imaging device so that ultraviolet light enters the sensor section.
2. A structure in which the thickness of the substrate side of a CCD solid-state imaging device is decreased so that light (ultraviolet light) is applied from the back surface of the substrate.
3. A structure in which a material, such as a fluorescent coating, which converts the wavelength of ultraviolet light into that of visible light is applied on the sensor section.
In the first structure described above, since ultraviolet light entering a section other than the sensor section cannot be detected, the effective aperture ratio of the sensor section is decreased. Additionally, since the sensor section is not provided with a protection film, a semiconductor layer in the sensor section is degraded unless moisture, etc. is blocked by a package or the like.
In the second structure described above, the ultraviolet light passing through the substrate is scattered and diverted to the adjacent pixels, resulting in a decrease in resolution. Additionally, since the substrate must be formed as thin as possible, as the thickness of the substrate is decreased, the fabrication cost is increased.
Moreover, since the CCD solid-state imaging device having the second structure is a frame transfer (FT) charge coupled device, in which an n-type buried channel is formed in a p-type substrate to constitute a sensor section which also acts as a transfer section, the dark current is increased. Also, an electronic shuttering behavior by draining electric charge toward the substrate is not allowed.
In the third structure, due to the slow response of the wavelength conversion from ultraviolet light into visible light, residual images easily occur.
Consequently, in the first to third structures, since it is an aim to provide sufficient sensitivity to ultraviolet light, the other characteristics have been deteriorated.